The present invention generally relates to ink delivery systems, and more particularly to a thermal inkjet printhead which is characterized by more efficient ink drop expulsion, controlled operating temperatures, high frequency operation, and reduced energy requirements. These goals are accomplished through the use of a novel internal design associated with the printhead as discussed in considerable detail below.
Substantial developments have been made in the field of electronic printing technology. A wide variety of highly-efficient printing systems currently exist which are capable of dispensing ink in a rapid and accurate manner. Thermal inkjet systems are especially important in this regard. Printing units using thermal inkjet technology basically involve an apparatus which includes at least one ink reservoir chamber in fluid communication with a substrate (preferably made of silicon [Si] and/or other comparable materials) having a plurality of thin-film heating resistors thereon. The substrate and resistors are maintained within a structure that is conventionally characterized as a "printhead". Selective activation of the resistors causes thermal excitation of the ink materials stored inside the reservoir chamber and expulsion thereof from the printhead. Representative thermal inkjet systems are discussed in U.S. Pat. Nos. 4,500,895 to Buck et al.; 4,794,409 to Cowger et al.; 4,771,295 to Baker et al.; 5,278,584 to Keefe et al.; and the Hewlett-Packard Journal, Vol. 39, No. 4 (August 1988), all of which are incorporated herein by reference.
The ink delivery systems described above (and comparable printing units using thermal inkjet technology) typically include an ink containment unit (e.g. a housing, vessel, or tank) having a self-contained supply of ink therein in order to form an ink cartridge. In a standard ink cartridge, the ink containment unit is directly attached to the remaining components of the cartridge to produce an integral and unitary structure wherein the ink supply is considered to be "on-board" as shown in, for example, U.S. Pat. No. 4,771,295 to Baker et al. However, in other cases, the ink containment unit will be provided at a remote location within the printer, with the ink containment unit being operatively connected to and in fluid communication with the printhead using one or more ink transfer conduits. These particular systems are conventionally known as "off-axis" printing units. Representative, non-limiting off-axis ink delivery systems are discussed in co-owned pending U.S. patent application Ser. No. 08/869,446 (filed on Jun. 5, 1997) entitled "AN INK CONTAINMENT SYSTEM INCLUDING A PLURAL-WALLED BAG FORMED OF INNER AND OUTER FILM LAYERS" (Olsen et al.) and co-owned pending U.S. patent application Ser. No. 08/873,612 (filed Jun. 11, 1997) entitled "REGULATOR FOR A FREE-INK INKJET PEN" (Hauck et al.) which are each incorporated herein by reference. The present invention is applicable to both on-board and off-axis systems which will become readily apparent from the discussion provided below.
Regardless of the particular ink delivery system under consideration, an important factor involves the operating efficiency of the printhead with particular reference to the resistor elements that are used to expel ink on-demand during printhead operation. The term "operating efficiency" shall encompass a number of different items including but not limited to internal temperature levels, operational speed, operating frequency (defined below), energy requirements, and the like. The resistor elements used for ink expulsion (which are produced from a number of compositions including but not limited to a mixture comprised of elemental tantalum [Ta] and elemental aluminum [Al], as well as other comparable materials) are discussed in considerable detail in U.S. Pat. Nos. 4,535,343 to Wright et al.; 4,616,408 to Lloyd; and 5,122,812 to Hess et al. which are all incorporated herein by reference. In accordance with their ability to selectively heat the desired ink compositions so that they can be expelled on-demand from the printhead, the resistors will reach very high peak temperatures, with the term "peak temperature" being defined to involve the maximum operating temperature of the resistor which is typically measured at the end of the electrical impulse that is used to "fire" the resistor and before any cooling occurs. For example, in conventional printhead systems (including those associated with the patents mentioned above), typical peak temperatures experienced by the thin-film resistors will be around 300-1250.degree. C., with such temperatures being reached when the resistor is activated/energized and being present when the "firing impulse" is terminated (before any cooling occurs). These high temperature values will at least partially influence the degree to which the resistors are able to cool down between sequential firing impulses (also characterized herein as "ink ejections".) Typically, the duration between successive firing impulses in a conventional thermal inkjet printhead will be about 20-500 microseconds (.mu.s), with the duration of each impulse being about 1-8 microseconds (.mu.s). Thus, only a minimal amount of time is available for the resistors to satisfactorily cool-down, with typical cool-down temperatures being about 60-85.degree. C. as discussed further below.
In accordance with the traditionally high resistor temperatures listed above and the minimal amount of available cool-down time, the overall operating frequency of the resistors in conventional printhead systems is limited. The term "operating frequency" is generally defined herein as the number of times per second that a given resistor is fired (or is able to fire) in a "black-out mode" (e.g. when the resistor is being used at a 100% rate to produce a solid zone of ink on the selected print medium). High operating frequency levels are desirable in a thermal inkjet printing system because they substantially improve printing speed which is usually expressed in pages per minute.
In conventional thermal inkjet systems including but not limited to those discussed in the U.S. patents listed above, (namely, U.S. Pat. Nos. 4,535,343 to Wright et al.; 4,616,408 to Lloyd; and 5,122,812 to Hess which are again incorporated herein by reference), each resistor is separated from the underlying substrate by an electrically-insulating layer of material. This layer (which is classified as a "dielectric" or insulator structure) is normally produced from silicon dioxide (SiO.sub.2) having a representative, non-limiting thickness of about 3.5 .mu.m (see U.S. Pat. No. 4,535,343 to Wright et al.) However, the thermal conductivity of this material does not vary in a significant manner during the temperature fluctuations which occur when the resistors thereon are operating. For reference purposes, the term "thermal conductivity" is defined to involve the heat flow across a surface per unit area per unit time, divided by the negative of the rate of change of temperature with distance in a direction perpendicular to the surface. This definition shall be applicable to the present invention and the various uses of "thermal conductivity" recited herein.
In accordance with the definition of thermal conductivity provided above, the higher the thermal conductivity of a material, the better the material is able to allow the passage of heat therethrough and thereby function as a heat transfer medium. The opposite situation exists in connection with materials having a lower thermal conductivity. Compositions with low thermal conductivity values prevent thermal energy (e.g. heat) from readily passing therethrough and are appropriately characterized as thermal insulators. This information is relevant to the present invention which will become readily apparent from the specific data disclosed in the Detailed Description of Preferred Embodiments section. When each of the resistors in a thermal inkjet printhead is activated using an electrical impulse (e.g. "signal") provided by the main printer unit, it generates sufficient heat to cause "ink bubble nucleation" and expulsion of the ink from the printhead. It is very important that the resulting "left over" heat generated by the resistor once the impulse has ended be rapidly dissipated from the resistor so that proper resistor "cool-down" can occur between impulses. However, between impulses and as the resistor is getting ready to receive the next impulse, it is likewise important that the heat dissipation characteristics of the system be minimized so that little if any heat will be dissipated therefrom when the resistor actually receives the next impulse. As a result, when the next impulse arrives, substantially all of the heat generated by the resistor will be imparted to the ink materials located above the resistor without "leakage" or dissipation of the heat through other parts of the printhead (especially the material layers located below the resistor.) In other words, the heat dissipation characteristics of the system should be "low" when the resistor "turns on" in order to impart substantially all of the heat to the ink (which reduces peak temperature requirements and energy consumption), with the heat dissipation characteristics of the system being "high" when the resistor "turns off" so that proper cooling can take place (which can improve operating frequency as noted above). A printhead which does not function in this manner is characterized by numerous adverse characteristics including but not limited to: (1) the need for increasingly-high resistor "peak" and/or steady-state temperatures in order to compensate for the thermal energy losses outlined above; (2) a reduced operating frequency caused by excessive resistor cool-down time between firing impulses or "ink-expulsions"; and (3) increased energy requirements which are necessary to achieve the higher resistor temperatures described herein. Regarding item (3), these increased energy needs are characterized by a higher "turn-on-energy" (or "TOE") which is defined as the electrical energy required by the resistor to cause an ink droplet (of the proper drop volume) to exit the orifice in the orifice plate (discussed below) at "saturated velocity". Saturated velocity generally involves the maximum possible velocity that the droplet can physically obtain for a given resistor architecture regardless of how much energy is applied to it.
It is particularly important that the thermal energy generated by the resistor elements be rapidly dissipated between successive ink ejections so that adequate resistor cool-down can occur as noted above. A lack of sufficient cool-down (e.g. to a preferred temperature of about 60-85.degree. C. or other comparable value) can cause multiple problems including but not limited to a reduction in operating frequency as previously discussed.
The use of silicon dioxide as a base layer in the printhead does little to control temperature-related problems at the minimum and maximum operating temperatures of the resistors. Instead, it contributes to high resistor peak and steady state temperatures and reduced operating frequency levels. When silicon dioxide is employed as the base layer between the resistors and the substrate, it cannot function as an effective "heat-dissipator" at the high temperatures which exist and remain immediately upon electrical impulse termination. Likewise, at the lower temperatures of the resistors between ink-ejection stages, silicon dioxide is insufficiently insulating to prevent heat loss when the next impulse is received, thereby allowing a substantial amount of heat to be diverted from the ink and dissipated out of the system when the resistor begins its next heating cycle.
Prior to the present invention, a need remained for a thermal inkjet printhead and method for producing the same which avoids the problems listed above. In accordance with the present invention, unique components, materials, and methods are described below which solve the foregoing difficulties in an effective manner. This goal is accomplished through the use of a novel base layer on which the resistor(s) are positioned which is made from a specialized material having a thermal conductivity that varies greatly with temperature in a positive manner. In particular, the claimed base layer has a high thermal conductivity at the elevated temperatures associated with resistor operation and thereby functions as an effective heat-dissipator when the impulses are terminated and the resistors are particularly "hot". This process facilitates proper resistor cool-down and increased operating frequencies. Simultaneously, the claimed base layer is characterized by a reduced thermal conductivity at the lower temperatures associated with the resistors when in an inactive state (e.g. between firing impulses). This reduced thermal conductivity allows the base layer to prevent undesired heat transfer or "leakage" therethrough when the resistors are first energized and "building up" sufficient heat for ink ejection. As a result, the TOE requirements of the system are reduced. Likewise, the claimed system also produces lower peak resistor temperatures as previously described.
In summary, the present invention involves a thermal inkjet printhead having a "self-adjusting" base layer designed to provide the benefits listed above. Also encompassed within the invention are the specialized chemical compositions which can be employed for this purpose, an ink delivery system using the claimed printhead, and a construction method for producing the printhead on a mass production scale. Accordingly, the invention represents a significant advance in thermal inkjet technology which ensures high levels of operating efficiency, excellent image quality, rapid throughput, and increased longevity.